Today they co-exist in Church Slavonic, with no pronunciation difference; and in Ukrainian, representing actual pronunciation differences. Other modern orthographies for Slavic languages eliminated one of the two letters in alphabet reforms of the 19th or 20th centuries: Russian, Macedonian, Serbian and Bulgarian languages use only ⟨И⟩, and Belarusian uses only ⟨І⟩.

Originally, Cyrillic ⟨И⟩ had the shape identical to the capital Greek letter Eta ⟨Η⟩. Later, the middle stroke was turned counterclockwise, resulting in the modern form resembling a mirrored capital Latin letter N ⟨N⟩ (this is why ⟨И⟩ is used in faux Cyrillic typography). But the style of the two letters is not fully identical: in roman fonts, ⟨И⟩ has heavier vertical strokes and serifs on all four corners, whereas ⟨N⟩ has a heavier diagonal stroke and lacks a serif on the bottom-right corner.

In roman and oblique fonts, the lowercase letter ⟨и⟩ has the same shape as the uppercase letter ⟨И⟩. In italic fonts, the lowercase letter ⟨И⟩ looks like the italic form of the lowercase Latin U ⟨u⟩. Both capital and small hand-written forms of the Cyrillic letter I look like hand-written forms of the Latin letter U.

Since the 1930s, ⟨и⟩ has been the tenth letter of the Russian alphabet, and in Russian, it represents /i/, like the i in machine except after some consonants (see below). In Russian, it typically denotes a preceding soft consonant and, therefore, is considered the soft counterpart to ⟨ы⟩ (which represents [ɨ]) but, unlike other "soft" vowels (⟨е⟩, ⟨ё⟩, ⟨ю⟩ and ⟨я⟩), ⟨и⟩ in isolation is not preceded by the /j/ semivowel. In Russian, the letter has been seen combined in the digraph ⟨ио⟩ (as were ⟨ьо⟩, ⟨їô⟩ and ⟨iо⟩) to represent ⟨ё⟩ before its existence around 1783. There still exist some apparent confusion in the transcription of some foreign words.

⟨И⟩ pronounced as [ɨ] in ⟨жи⟩ (sounds like ⟨жы⟩ [ʐɨ]), ⟨ши⟩ (sounds like ⟨шы⟩ [ʂɨ]) and ⟨ци⟩ (sounds like ⟨цы⟩ [t͡sɨ]), because in Russian, the sound [i] is inarticulable after "zh" ⟨ж⟩, "sh" ⟨ш⟩ and "ts" ⟨ц⟩.

In Kazakh, I with diaeresis is used in loanwords. For native words, ⟨И⟩ is used.

In Ukrainian and Belarusian, the sound /i/ is represented by another letter ⟨і⟩, sometimes called Ukrainian I, removed from the modern Russian alphabet.

Ukrainian and Belarusian ⟨і⟩ sounds like Russian ⟨И⟩ [i], but a clearly distinct sound [ɪ] is represented by ⟨и⟩ in Ukrainian, which differs only slightly from Russian ⟨ы⟩ and is perceived as ⟨ы⟩ by a Russian speaker.

The vowel represented by ⟨и⟩, as well as almost any other Slavonic vowel, can be stressed or unstressed. Stressed variants are sometimes (in special texts, like dictionaries, or to prevent ambiguity) graphically marked by acute, grave, double grave or circumflex accent marks.

Special Serbian texts also use ⟨и⟩ with a macron to represent long unstressed variant of the sound. Serbian ⟨и⟩ with a circumflex can be unstressed as well; then, it represents the genitive case of plural forms to distinguish them from other similar forms.

Modern Church Slavonic orthography uses the smooth breathing sign (Greek and Church Slavonic: psili, Latin: spiritus lenis) above the initial vowels (just for tradition, as there is no difference in pronunciation). It can be combined with acute or grave accents, if necessary.

None of those combinations is considered as a separate letter of respective alphabet, but one of them (⟨Ѝ⟩) has an individual code position in Unicode.

⟨И⟩ with a breve forms the letter ⟨й⟩ for the consonant /j/ or a similar semivowel, like the y in English "yes." The form has been used regularly in Church Slavonic since the 16th century, but it officially became a separate letter of alphabet much later (in Russian, only in 1918). The original name of ⟨й⟩ was I s kratkoy ('I with the short [line]'), later I kratkoye ('short I') in Russian. It is known similarly as I kratko in Bulgarian but as Yot in Ukrainian.

Cyrillic alphabets of non-Slavic languages have additional ⟨и⟩-based letters, like ⟨Ӥ⟩ or ⟨Ӣ⟩.